Solid imaging generally involves the formation of three-dimensional objects according to computer commands based on a computer aided design ("CAD") or other three-dimensional representation of the object. One solid imaging technique recently developed is stereolithography which is described in U.S. Pat. Nos. 4,575,330 and 5,184,307, both of which are incorporated by reference as if fully set forth herein. Appearing below is a summary of the basic steps of a stereolithographic embodiment:
1. Generation of a three-dimensional object design in a CAD system and storage of the design data in a CAD file; PA1 2. Compiling data from the CAD file into numerous thin "slices" each representing a thin cross-sectional layer of the three-dimensional object; PA1 3. Transfer of the compiled CAD data to a StereoLithographic Apparatus ("SLA"); PA1 4. Coating a layer of building material adjacent to a previously formed object cross-section in preparation for forming a subsequent object cross-section. The building material layer is preferably uniformly coated at an appropriate thickness so that the subsequently formed object cross-section meets tolerance requirements; PA1 5. Selectively exposing the building material layer to synergistic stimulation to solidify or otherwise physically transform the building material layer at those locations which collectively represent the object cross-section to be formed; PA1 6. Repeating steps (4) and (5) to alternately form successive building material layers and object cross-sections until the three-dimensional object is formed; and PA1 7. Post processing the newly-formed object by removing residual building material clinging to the object, removing the object from the platform on which it was formed, exposing the object to additional synergistic stimulation to ensure complete solidification of the building material and removing supports.
Building materials typically used in solid imaging may exhibit fluid-like characteristics but solidify or otherwise physically transform in response to synergistic stimulation. The fluid-like characteristics facilitate dispensing a building material layer adjacent to a previously formed object cross-section, as well as smoothing the building material layer surface in preparation of forming the next object cross-section. Depending on the coating technique used, suitable materials include transformable liquids such as thermally polymerizable resins, photopolymerizable resins, a first part of a two-part epoxy, sinterable powders, bindable powders or combinations thereof and the like. Liquid materials may also contain inert filler materials.
Various forms of synergistic stimulation may be used as long as the building material is responsive to the synergistic stimulation. These include certain wavelengths of electro-magnetic radiation, such as infrared radiation, visible radiation and ultraviolet radiation. Other forms of synergistic stimulation which may be used are particle beams, reactive chemicals dispensed onto the building material such as a photoinitiator (the second element of a two-part epoxy), binder materials, and the like.
The design data, representative of the three-dimensional object can be obtained from various sources including CAD data, CAT scan data, manually programmed data, and data derived from techniques for scanning physical objects. If this data is initially in layer form, the compilation process may be reduced to creating appropriate layer fill data. However, additional compilation may be desired or required to transform the data into proper form to meet accuracy, process or other requirements such as how supports will be built along with the object. The procedures and apparatus described in U.S. Pat. Nos. 5,182,055, 5,184,307, 5,192,469, 5,209,878, 5,238,639, 5,256,340, 5,273,691 and 5,321,622, 5,345,391, and U.S. patent application Ser. No. 08/233,026, pending, and Ser. No.08/233,027, pending, both filed Apr. 25, 1994, address the generation of appropriate layer data. All of these patents and patent applications are incorporated by reference as if fully set forth herein. Also incorporated by reference as if fully set forth herein, is the publication entitled Rapid Prototyping & Manufacturing: Fundamentals of Stereolithography, First Edition, authored by Paul F. Jacobs, Ph.D., and published by the Society of Manufacturing Engineers, Dearborn, Mich., in 1992.
The current invention is directed primarily to step (4) above, i.e., coating a building material layer adjacent to a previously formed object cross-section in preparation for forming a subsequent object cross-section. Several approaches have been used in the past to perform this coating step, most often with a building material comprising a liquid photopolymerizable resin. However, these prior approaches have resulted in varying degrees of layer accuracy and nonuniformity, and/or have required excessive time to form the coatings, these problems have the following ramifications:
First, it is important that the building material layer is uniform and of appropriate thickness so that upon solidification, the resulting object cross-section exhibits dimensional accuracy. Indeed, the accuracy of the successive building material layers directly impacts the accuracy of the final object in view of potential misplacement of object features upon exposure to synergistic stimulation and potential accumulated errors which may result from errors on successive layers.
Second, it is desirable to minimize the time required to form a building material layer because the cumulative coating time of the successive layers represents a significant portion of the overall object build time. Indeed, photopolymer resins exhibit slow flow velocities due to viscosity and surface tension. If driven only by gravity, imperfections in photopolymer building material layer surfaces can take prohibitively long time periods to relax or otherwise become uniform with the rest of the building material layer surface. This in turn increases object build time, reduces machine throughput, and reduces the cost effectiveness of solid imaging.
Third, the extent of inaccuracy and nonuniformity of the building material layer as well as the amount of time necessary to form it may vary with the geometry of previously formed cross-sections. Accordingly, automated coating of building material layers is difficult because there may be no set correction parameters that might otherwise be used if coating inaccuracies were constant.
A description of several previous approaches is set forth in the following U.S. Patents and Patent Applications, the disclosures of which are all incorporated by reference as if fully set forth herein:
1) U.S. patent application Ser. No. 07/414,200, now abandoned, by Hull, et al filed Sep. 28, 1989, and its continuation Ser. No. 08/230,443 filed Apr. 20, 1994, now U.S. Pat. No. 5,447,882, are directed to covering the building material layer surface with a film which is then peeled from the surface. Before or after peeling, the surface is exposed to synergistic stimulation to form the next object cross-section.
2) U.S. patent application Ser. No. 07/495,791, now abandoned, by Jacobs et al filed Mar. 9, 1990, and its continuation Ser. No. 08/198,655, now abandoned, filed Feb. 18, 1994, are directed to the use of vibrational energy applied directly to the building material layer surface or to a previously formed object cross-section to decrease the time required for surface imperfections to vanish or level out to a tolerable level.
3) U.S. Pat. No. 5,174,931 issued to Almquist, et al. and its continuation Ser. No. 08/146,562 filed now abandoned Nov. 2, 1993 are directed to, among other things, using a member such as a doctor blade, to smooth or spread a coating of building material over a previously formed cross-section of the object.
4) U.S. Pat. No. 5,096,530 issued to Cohen, et al. is directed to forming a building material layer which is supported by a frame and the force of surface tension. The layer is then laid above a previously formed object cross-section.
5) U.S. Pat. No. 5,071,337 issued to Heller, et al. and its continuation-in-part Ser. No. 08/299,879 filed Sep. 1, 1994, now abandoned are directed to, among other things, using a dispensing device such as an applicator bar to form uniform building material layers.
The doctor blade approach listed above typically involves sweeping a bar or other device across the surface of a building material layer thereby smoothing it. Though this may reduce coating time, other problems remain such as those associated with leading edge bulge, trapped volumes, scoop-out and other problems described in previously incorporated U.S. Pat. No. 5,174,931.
Other coating approaches have been suggested beyond those listed above. An electrically charged or uncharged counter-rotating roller which spreads a mound of powder into uniform layers is disclosed in PCT Patent Application No. PCT/US87/02635, Publication No. WO 88/022677 by Deckard, and in U.S. Pat. No. 4,938,816 issued to Beaman et al. However, the roller disclosed therein is generally not suited for use with liquid building materials because liquids may cling to the roller unlike the powders described in the above references which are instead ejected in front of the roller. This clinging action may also cause building material to be carried over the roller and redeposited behind it thereby creating a nonuniform building layer. Furthermore, liquid mounds also tend to sink or spread out into previously dispensed volumes of unsolidified liquid. In any event, the Deckard and Beaman references do not address how such a roller might be used with liquid building materials.
Several basic aspects of using a dispensing slit or curtain coater in a stereolithographic process are disclosed in Japanese Patent Application 59-237054, laid open to the public as Japanese Publication 61-114817(A) on Jun. 2, 1986, filed by Morihara et al. The slit coater remains stationary as the container of liquid building material is moved back and forth beneath it. The slit coater dispenses building material having a thickness equal to that of the desired solidified object cross-section. However, Morihara's slit coater is not suitable for producing high-resolution objects for at least the following reasons.
First, forming building material layers having a thickness equal to that of the desired object cross-section does not account for the shrinkage which typically occurs as the building material solidifies. This in turn leads to inaccuracies in the vertical dimensions of the object, formation of non-planar object cross-sections especially in transitional regions between supported and unsupported portions of a cross-section, and uncontrolled positioning of the working surface.
Second, Morihara's slit coater does not account for the volumetric difference of material dispensed when the container moves at constant velocity versus when it accelerates and decelerates near the ends of its line of travel. This results in a nonuniform thickness across the building material layer.
Third, Morihara's slit coater cannot dispense material at locations of the container which are inaccessible to the slit coater. This either reduces the accuracy of the overall coating formed or the usable working area of the container.
Fourth, Morihara's slit coater does not recognize that in certain stereolithographic embodiments, one must coat a building material layer over the entire surface of the liquid bounded by the container before the building material layer achieves the desired thickness. This is because when building material is dispensed in regions that are not closely supported by solidified material, the building material will not simply remain at the surface of the liquid in the container. Instead, it serves to raise the liquid level in the entire container thereby decreasing the thickness of the building material layer at the point it was just dispensed at. Only after material has been dispensed over all such unsupported regions will the building material surface level reach the desired level. In certain circumstances however, such as when coating very thin building material layers on the order of 0.004 inches or less, one may ignore this problem.
Fifth, the building material in Morihara's container is likely to shift due to the repeated to and fro container motion. Such shifting would likely result in nonuniform coating thicknesses and/or increased layer formation times. In fact, even if the container is moved to and fro at moderate speeds, the material in the container may slosh out of the container. For all the foregoing, it appears that Morihara does not disclose an apparatus or method to rapidly and accurately recoat building material layers.
Beyond the problems of the particular approaches discussed above, other problems involve the dispensing of a known quantity of building material, or avoiding the accumulation of small errors into large cumulative errors as successive layers are coated. Accordingly, there is a need in the RP&M art for methods and apparatus which overcome the problems discussed in this background section as well as other problems.